Currently, a nonaqueous electrolyte secondary battery represented by a lithium ion secondary battery has been widely used for consumer applications such as small portable devices because of its high energy density. In a general lithium ion secondary battery, a transition metal oxide such as LiCoO2 has been used as a positive electrode active material, a carbon material such as graphite has been used as a negative electrode active material, and a nonaqueous electrolyte obtained by dissolving an electrolyte salt such as LiPF6 in a nonaqueous solvent such as a carbonic acid ester has been used as an electrolyte solution.
Moreover, lithium titanate that allows absorbing/desorbing reaction of the lithium ions to occur at an electric potential relative to that of lithium of about 1.5 V, the electric potential being nobler when lithium titanate is compared with carbon materials, has been proposed as a negative electrode active material.
In Patent Literature 1 for example, a nonaqueous electrolyte secondary battery using lithium titanate as a negative electrode active material and using, as a nonaqueous solvent for nonaqueous electrolytes, a mixed solvent of a cyclic carbonate such as propylene carbonate and a chain carbonate such as diethyl carbonate has been proposed.
Moreover, in Patent Literature 2 for example, a nonaqueous electrolyte secondary battery using lithium titanate as a negative electrode active material and using, as a nonaqueous solvent for nonaqueous electrolytes, a mixed solvent of a cyclic carbonate, dimethyl carbonate, and an ethyl methyl carbonate has been proposed.